(a) The Invention
The invention relates to novel bearings and methods for making bearings. An increased thrust area, particularly useful if one or more components is polymeric, may be achieved thereby allowing a reduction in thrust pressure.
(b) Background Information
Bearings comprising rolling or sliding elements, such as balls or rollers, and at least one race comprising a surface upon which the rolling or sliding element can exert thrust are known to support or to reduce the wear of rotating shafts, or to transfer loads between rotating and stationary members with a minimum of friction. Rolling or sliding elements and races have each been made of a variety of materials such as metals (notably steel), ceramics and polymers. The Standard Handbook for Mechanical Engineers, Eighth Edition (1978), pages 8-136 to 8-142, the disclosure of which is incorporated herein by reference, provides a discussion of various bearings. Where the bearing is to be used near foods, water, corrosive materials etc., it may be desirable that the rolling or sliding elements and race comprise a polymeric material rather than a metal. Such a polymeric material may however be softer than a metal and may be able therefore to withstand a smaller maximum thrust. It is then highly desirable to provide a large thrust area. A larger thrust area will in general, however, require that the race conform to the shape of the rolling element to a greater extent than otherwise. This may in turn mean that the race traps the rolling or sliding element to a greater extent. (A ball bearing whose outer and inner races have flat bearing surfaces would not ordinarily trap the balls at all; the more the thrust area is increased by curving those surfaces around the balls, the more the balls are trapped.) This increase in the extent to which the balls are trapped clearly increases the difficulty in assembling the bearing, since the restriction that prevents the rolling or sliding elements leaving the race tends also to prevent their insertion. One way of facilitating assembly of a bearing is to provide sufficient tolerance between the rolling or sliding elements and other parts (for example inner and outer ring races) such that one part can be moved with respect to another creating a gap wider than the rolling or sliding elements to be inserted. However, this technique limits the number of elements that may be inserted, and therefore although the thrust area per element may be large, the total is small. Also the elements so inserted may easily fall out. Another method is to make the race in pieces and assemble those pieces around the rolling elements with screws, adhesives, or the like, or by interference fit. A problem with that type of race, however is its tendency to fail under a load and the difficulty of achieving precise tolerances. A further method involves providing a side tunnel into the race, that can admit the elements and then be closed. The resulting thrust area is, however, likely not to be smooth, causing abrasion of the elements.
We have now discovered that the phenomenon of dimensional-recovery may be used to overcome the otherwise inconsistent demands of ease or even possibility of assembly, and high thrust area or resistance to falling apart. We have discovered that the close tolerances required of the components of a bearing may be retained after the shaping, expansion and shrinking steps characteristic of the formation of dimensionally-recovered, particularly heat-recovered, articles.
Although heat recovery has been used in connection with bearings, it has not been used to solve the problems we address. The following documents may, however, be mentioned.
In French patent No. 2,408,362-A, a heat recoverable collar holds an inner bearing face around a skipping rope. Rolling elements and an outer race are then assembled in a conventional manner. The purpose of the bearing is of course to allow a freely rotatable connection to a handle so that the rope does not become twisted in use.
In Canadian patent No. 908,404 an adhesive which bonds a PTFE surface to an intermediate member around a rolling element, contracts on heating and therefore releases the rolling elements, allowing them to move freely.
In Canadian patent No. 887,365-A a heat shrinkable tape is applied to pressurize an epoxy tape which coats an outer surface of a metallic bearing shell in order to insulate it electrically. This is done to protect the bearing from splitting due to electrical current which may otherwise pass through it.